Process for making a gas from solid fuels and burning the gas in a close coupled combustor to produce clean heat

ABSTRACT

A process is described for gasifying solid fuels at a constant rate with a small regulated amount of primary air followed by combustion of the gas so formed with a larger regulated amount of secondary air to produce a clean hot combustion gas which is used for process heat, space heating or cooking.

TECHNICAL FIELD

[0001] This invention relates to a method for converting biomass and other solid fuels to gas and close coupled combustion of that gas for process heat, cooking and other applications.

RELATED PATENTS

[0002] TABLE 1 Prior art of solid fuel combustion for cooking and other uses PATENT TITLE US3982522 Portable Forced draft Cooker US4209006 Barbecue unit US4392477 Solid fuel burning stove US4475529 Solid fuel burning stove with exterior rear wall baffle US4516561 Portable battery powered blower apparatus for fanning charcoal or other fuel US4616627 Forced draft stove US4730597 Biomass stove US4823684 Pellet-fired barbecue US5024208 Portable stove US5031602 Convertible portable cooking apparatus US5076258 Automatic food cooker with a tubular frame constituting hot and cold forced air conduits US5168860 Barbeque US5429110 Mobile pellet stove with thermal barrier and ventilated firepot

[0003] TABLE 2 Fixed Bad gasification patents US04919686 Process for the pyrolytic production of synthetic gas US05611289 Gasifier for biomass waste and related volatile solids US05427077 Apparatus for delivering a volatile combustible vapor and atmospheric air mixture to internal combustion engines US04764185 Gasifier apparatus US04586442 Gasifier method and apparatus US05823122 System and process for production of fuel gas from solid biomass fuel and for combustion of such fuel gas US05133780 Apparatus for fixed bed coal gasification US04848249 System and process for conversion of biomass into usable energy US05145490 Process for fixed bed coal gasification

[0004] These patents describe processes for converting solid fuels to gas but not close coupled for direct heating applications.

BACKGROUND OF THE INVENTION

[0005] Wood and other forms of biomass have been used for cooking since the beginning of Civilization with low efficiency and poor heat transfer. When gas became available in the early 1800s one of the first uses was for cooking because the gas flame is clean, intense, and the cooking level easily controlled. However, gas and electric cooking require a sophisticated infrastructure for gas or power delivery and over half the population of the world is still cooking with biomass fires that are slow, inefficient and a hazard to health. Wood and coal fire emissions are the cause of millions of premature deaths each year in developing countries.

[0006] This invention relates generally to the field of combustion and gasification, and more particularly to a process and various devices for gasification of biomass for producing process heat or for clean smokeless cooking. Fire has been the necessary companion and problem of our developing civilization. Solid fuels such as coal and biomass are difficult to burn in a clean fashion and

[0007] Industrial development was accelerated when it was found possible about 1800 to gasify biomass and coal and then burn the gas in a clean, controlled manner. Later I have learned to rely on natural gas, liquefied gases and liquid fuels to accomplish these tasks without needing gasification, but oil and gas are not renewable resources and are not easily available to half the population of the world.

[0008] Household cooking particularly has suffered from incomplete combustion of solid fuels and been benefited first by the introduction of manufactured gas and now by natural gas, 1 pg or electric cooking. However, these modern advances are not available to half the households of the world.

[0009] In developing countries the three stone stove is still widely used in rural households with consequent major health problems for women and children from the smoke and carbon monoxide emissions. In the towns and cities clean charcoal cooking is more prevalent, but the manufacture of charcoal wastes ⅔ of the energy in the wood and creates massive greenhouse gases on a global scale.

[0010] This invention uses gasification technology to create a clean burning gas and then provide sufficient air to burn the gas to produce a lean intense flame for efficient cooking similar to that provided by natural gas or liquefied petroleum gases (propane, butane).

DISCLOSURE OF THE INVENTION

[0011] It will be evident to those skilled in the art that this invention combines some of the elements of combustion and gasification in a new and useful manner to form an integral gasifier-burner producing hot gases for various uses such as cooking, baking, brick making, home heating, drying and other uses where a reliable supply of gas is required.

[0012] Fixed bed gasifiers are generally divided into the classes updraft and downdraft, but these should be more properly called counter-flow and co-flow, depending on whether air moves against the flow of fuel or in the same direction. In updraft gasifiers, air passes up through a bed of burning charcoal generating heat which then pyrolyses the incoming fuel. They generate more tar than gas.

[0013] In conventional downdraft gasifiers it is necessary to force or draw air down through the fuel. The hot gas must then be conveyed to the use point. Such gasifiers were widely used during WWII to power vehicles and for power generation.

[0014] I have developed a new form of co-flow gasification in which air enters a fuel bed from below and gasification starts at the top of the bed and proceeds through the fuel. The gas is generated at the top of the bed, rather than at the bottom as in conventional downdraft gasification. This new mode has the advantage that the direction of hot gas flow (up) is the same as the natural flow caused by natural convection so that the gasifier can operate with natural convection alone. However, forced convection increases the rate of gas production and has other advantages discussed below.

[0015] This invention combines a top burning gasifier with a close-coupled mixing-combustion chamber to produce useful heat. The gasifier includes a source of primary gasification air which unites with the fuel in a controlled gasification zone, producing a combustible gas according to

Solid Fuel+Primary Air→Combustible gas (700° C.) GASIFICATION STAGE

[0016] The combustible gas is then mixed with secondary air to produce hot combustion products for any heat application according to

Combustible gas+Secondary air→Hot flue gases COMBUSTION STAGE

[0017] The invention comprises supplying controlled primary air below a mass of fuel in which a flaming pyrolysis zone generates charcoal and a combustible gas and moves slowly down through the mass coupled to a stable combustion zone in which controlled secondary air produces essentially complete combustion and very clean hot gases useful in many ways. The air supply can be by natural or forced convection.

DRAWING FIGURES

[0018]FIG. 1 shows a process diagram of a close coupled top burning gasifier and combustion heating system.

[0019]FIG. 2 shows a diagram of a natural convection close coupled gasifier-combustion heating device.

[0020]FIG. 3 shows a forced draft embodiment of this invention using a close coupled gasifier-combustor suitable for cooking.

[0021]FIG. 4 shows one embodiment of a blower operated forced convection close couple gasifier-combustor heating device suitable for cooking.

DETAILED DESCRIPTION OF DRAWINGS

[0022] In FIG. 1 a controlled quantity of air for gasification (1) passes up through first the gasifier section (2) of the combustor through raw fuel (3), a flaming pyrolysis section (4) and charcoal resulting from the pyrolysis (5), and is mixed with a controlled quantity of combustion air (8) in the combustion section (7) producing a clean combustion gas (9) which can be used for process heat, drying, cooking, space heating and other purposes.

[0023]FIG. 2 shows a natural convection close coupled gasifier/combustor device in which controlled air enters the device at (11), passes through unburned fuel (12) to the flaming pyrolysis section (13) and through the charcoal (14). The pyrolysis gas is (15) mixed with controlled secondary combustion air (16) to produce a clean hot gas stream which can be used for process heat, drying, cooking, space heating and other purposes.

[0024]FIG. 3 shows a forced air close coupled gasifier/combustor device in which controlled air (21) is supplied to a plenum chamber (23) by any source of compressed air such as a bellows or bladder and then passes up through the flaming pyrolyis zone (24) which produces charcoal and a combustible gas 25 which mixes with a controlled quantity of air at (26) to produce a clean hot gas stream which can be used for process heat, drying, cooking, space heating and other purposes.

EXAMPLES Example 1

[0025] A gasifier-combustor was constructed from insulating materials to form a useful heating device as shown in FIG. 2. In a typical natural draft run, stable gasification at 12 g/min was achieved. A pan containing 1 liter of water was placed on the stove at 11 min and reached boiling at 19 minutes. Water was then simmered for 16 minutes. 150 g of charcoal remained. The efficiency of heating of the water was estimated to be 35% based on the amount of water boiled and the fuel consumed during that time.

Example 2

[0026] A gasifier-combustor was constructed based on upward flow of gases through a packed bed of solid fuel such as coal or biomass as shown in FIG. 2.

[0027] The fuel section was filled with 125 g of hardwood chips. The fuel was ignited on top, primary air was passed through the fuel bed at a rate of 3 kg/hr forming a combustible gas. The gas was then mixed with secondary air in the jets. The resulting mixture burned above the fuel bed. The combustion products were then used to heat a container of water to measure heat transfer. The efficiency of heat transfer from the flame to the water was measured as 41% of the energy in the fuel consumed.

[0028] The gasifier-combustor was operated in a closed room with no smell of burning gases. The water container showed no sign of soot from incomplete combustion.

[0029] The wood chips burned for 22 minutes, producing enough heat to boil water in 4 minutes and keep it boiling. The useful heat to the pan was measured as 3 kW.

Example 3

[0030] The forced draft gasifier/combustor shown in FIG. 4 was powered by an AA cell and 1.5 volt blower. It was filled with 300 g of wood pellets and ignited on the top. The pellets burned for 50 minutes producing an average power of 2.8 kW. There was no smell of burning wood in a 1000 ft³ closed space.

Example 4

[0031] A charge of 80 g of hardwood chips was loaded into the gasifier chamber and a few chips soaked in alcohol were placed on the top to promote quick, clean starting. The lighter chips were ignited with a match and the blower was turned on at low speed. After 1 minute the blower was turned to full speed and a pan containing ½ liter of water was placed over the flame. The water came to a boil in 3 minutes and continued to boil vigorously for 12 minutes when the fuel was exhausted. At the end of the run there were 280 ml of water remaining. The efficiency of the stove was calculated to be 43% based the heat of vaporization of the water. A quantity charcoal equal to 24% of the starting fuel was produced. 

What is claimed is:
 1. A process for providing clean combustion of solid fuels comprising a means of delivering a relatively small quantity of primary air to the bottom of a quantity of solid fuel ignited at the top, said combustion forming a flaming pyrolysis zone producing charcoal and combustibly gas and moving slowly down through the solid fuel convering all of it to gas and charcoal at a constant rate, and a regulated quantity of secondary air then mixing with the gases to produce substantially complete combustion of the gases which produce clean hot combustion gases which are used for process heat, space heating or cooking.
 2. A device based on the process of claim 1 in which air passes by natural convection up through a fuel mass in which a slaming pyrolysis zone initially ignited at the top, passes down through the fuel mass relasing hot pyrolysis gases which rise by natural convection through the charcoal resulting from the said flaming pyrolysis and mix with regulated secondary air above the fuel mass creating a clean combustion flame which generates hot combustion gases which to produce clean hot combustion gases which are used for process heat, space heating or cooking.
 3. A device based on the process of claim 1 in which forced air is provided to a plenum chamber which in turn supplies a small regulated quantity of primary air which passes up through the unburned fuel mass, reacting with the downward moving flaming pyrolysis zone which releases gases and makes char at a constant rate which said gases are then mixed with a regulated quantity of secondary air in a combustion chamber to produce clean hot combustion gases which are used for process heat, space heating or cooking.
 4. A device based on the process of claim 1 in which an electric blower powered by battery or line power supplies regulated primary and secondary air to produce a clean flame and hot combustion gases which are used for process heat, space heating or cooking. 